1. Field of the Invention
This invention relates to a superconducting magnet apparatus for use in a magnetic resonance tomographic diagnosing apparatus (i.e. MRI, NMR), which has a heat insulating structure such as a coolant circulating path.
2. Description of the Related Art
Most magnetic resonance tomographic diagnosing apparatuses obtain a magnetic field of required high intensity by means of superconducting magnets. A magnet apparatus employed in such a magnetic resonance tomographic diagnosing apparatus a has a magnetic field whose axis is substantially at right angles to the line of gravity (i.e. arranged in the horizontal direction), since a bed must be put into and out of the magnetic field, together with a patient lying thereon. The magnet apparatus is generally annular or cylindrical, and comprises a superconducting coil having an axis substantially perpendicular to the line of gravity, a cylindrical vessel containing liquid helium for cooling the coil, and a space (for accommodating a patient and the like) which has a shape similar to the superconducting coil, and houses the coil and vessel. The vessel is generally made of a non-magnetic material such as stainless steel. The vessel and super heat insulating layers covering the entire inner and outer surfaces form a cryostat.
To keep the superconducting coil in a stable superconducting state, it is desirable that the entire coil is always immersed in liquid helium. It is also desirable to resupply liquid helium after at least one month elapses since the last supply. To meet these, a great amount of liquid helium needs be stored in a position higher than the upper surface of the superconducting coil. To store a great amount of liquid helium in such a position, conventional superconducting magnetic apparatuses have been designed such that the above-described receiving space has an outer diameter and an inner diameter much smaller than the former.
This conventional apparatus, however, has the following problems:
Since its liquid helium vessel is obtained by setting large the difference between the inner and outer diameters, a great amount of liquid helium necessarily exists also in a position lower than the upper surface of the coil. Little of the liquid helium stored in the lower position is used effectively, i.e., liquid helium of an amount more than required is stored in the vessel. If the superconducting coil is quenched to a normal conducting state, the stored liquid helium will be abruptly vaporized. There is a dangerous case of a conventional apparatus whose liquid helium vessel was damaged due to an increase in pressure within the vessel. Further, from an economical point of view, this is disadvantageous to that scanner for scanning the entire body of a patient which requires as much as 200-300 l of expensive liquid helium.
To eliminate the above disadvantage, it is considered to reduce the amount of liquid helium stored in a position lower than the upper surface of the superconducting coil, by arranging the vessel to have an axis extending in a plane higher than that in which the axis of the coil extends. In this case, however, since the vessel made of a conductive material is asymmetry with respect to the axis of a gradient coil, which is excited pulsively, an ununiform eddy current will be caused along the vessel, providing distorted tomograms.
In summary, the conventional superconducting magnetic apparatus is disadvantageous in view of economy and safety, since it stores a coolant of an amount more than required, and since it may be damaged when an abnormality such as quenching of the superconducting coil occurs.